The present invention relates generally to liquid crystal display devices, and more particularly to a system and method for generating on the display screen of a liquid crystal display device pixels accurately shaded in correspondence with a digital or analog video signal.
Liquid crystal displays (LCDs) are commonly used in devices such as portable televisions, portable computers, control displays, and cellular phones to display information to a user. LCDs act in effect as a light valve, i.e., they allow transmission of light in one state, block the transmission of light in a second state, and some include several intermediate stages for partial transmission. When used as a high resolution information display, as in one application of the present invention, LCDs are typically arranged in a matrix configuration with independently controlled pixels. Each individual pixel is signaled to selectively transmit or block light from a backlight (transmission mode), from a reflector (reflective mode), or from a combination of the two (transflective mode).
An LCD pixel can control the transference for different wavelengths of light. For example, an LCD can have pixels that control the amount of transmission of red, green, and blue light independently. In some LCDs, voltages are applied to different portions of a pixel to control light passing through several portions of dyed glass. In other LCDs, different colors are projected onto the pixel sequentially in time. If the voltage is also changed sequentially in time, different intensities of different colors of light result. By quickly changing the wavelength of light to which the pixel is exposed an observer will see the combination of colors rather than sequential discrete colors. Several monochrome LCDs can also result in a color display. For example, a monochrome red LCD can project its image onto a screen. If a monochrome green and monochrome blue LCD are projected in alignment with the red, the combination will be full color.
The monochrome resolution of an LCD can be defined by the number of different levels of light transmission that each pixel can perform in response to a control signal. A second level is different from a first level when the user can tell the difference between the two. An LCD with greater monochrome resolution will look clearer to the user.
LCDs are actuated pixel-by-pixel, either one at a time or several simultaneously. A voltage is applied to each pixel and the liquid crystal responds to the voltage by transmitting a corresponding amount of light. In some LCDs an increase in the actuation voltage decreases transmission, while in others it increases transmission. When multiple colors are involved for each pixel, multiple voltages are applied to the pixel at different positions or times depending upon the LCD. Each voltages controls the transmission of a particular color. For example, one pixel can be actuated to allow only blue light to be transmitted while another allows only green. A greater number of different light levels available for each color results in a much greater number of possible combination colors.
Converting a complex digital signal that represents an image or video into voltages to be applied to the pixels of an LCD involves circuitry that can limit the monochrome resolution. The signals necessary to drive a single color of an LCD are both digital and analog. It is digital in that each pixel requires a separate selection signal, but it is analog in that an actual voltage is applied to the pixel to determine light transmission. The conversion from a bit-representation of the desired light transmission, as communicated in the image or video signal, to an actual voltage that controls the light transmission can introduce errors that reduce the monochrome resolution of the LCD. For example, if a Digital-to-Analog Converter (DAC) takes as an input a bit-representation of voltage that includes 256 voltage levels and outputs voltages between 0 and 16 volts, the ideal output levels would differ by 62.5 millivolts (mV). If that DAC has an error of +/xe2x88x9240 mV, a user is likely to confuse the light level that corresponds to a particular voltage with the light levels of its neighbors and the number of effectively different levels of monochrome display is halved. Taking into account the errors that exist for each color of light being controlled in a pixel of the LCD reveals that the number of different combinations can be severely reduced. Further, if multiple DACs are used to drive the LCD, each DAC controlling a fraction of the pixels; then inaccuracies DAC-to-DAC can lead to distortions of the bit representation of the desired light transmission.
The present invention is directed to a system and method for actuating a liquid crystal display.
In one embodiment of the present invention, a matrix of liquid crystal pixels is provided. A digital-to-analog (DAC) converter is coupled to the matrix and produces an output voltage that can be applied to one or more pixels in the matrix. The DAC receives a calibrated multi-bit digital input and generates its output voltage to correspond to the digital input. A reference voltage generator generates at least two reference voltages at an output that is coupled to one input of a comparator. The other input of the comparator is coupled to the DAC output. When the comparator is activated, it outputs one of two signals: a first signal when one input receives a higher voltage; and a second signal when the other input receives a higher voltage. A digital calibrator has at least one output and at least one input. The digital calibrator is adapted to receive an uncalibrated multi-bit digital signal at the input. The digital calibrator is also adapted to modify the uncalibrated multi-bit digital signal based at least in part on the output of the comparator to produce the calibrated digital signal. The digital calibrator applies the calibrated multibit digital signal at the output. The output of the digital calibrator is coupled to the input of the digital-to-analog converter.
In a more specific embodiment of the present invention, the DAC comprises first and second circuits. The first circuit is adapted to receive a multi-bit digital signal as an input and to output a current corresponding to the digital signal. The second circuit receives the current generated by the first circuit and outputs a voltage corresponding to the current. In one embodiment the second circuit includes an operational amplifier.
In another embodiment of the present invention, the system includes at least a second DAC. The system also includes a multiplexer circuit coupled to the comparator, the reference voltage generator, and the DACs. The multiplexer circuit is adapted to receive the at least two reference voltages and to apply one of the reference voltages to one input of the comparator. The multiplexer circuit is also adapted to receive the output voltages of the DACs and to apply one of the output voltages to the other input of the comparator. In a more specific embodiment, the multiplexer circuit includes two multiplexers. The first multiplexer receives the reference voltages and the second receives the DAC output voltages. In a more specific embodiment, the digital calibrator is coupled to the multiplexer circuit and controls the output of the multiplexer circuit.
A technical advantage of the present invention is that it controls the light level of pixels of a liquid crystal display. Another technical advantage of the present invention is that it calibrates digital input to take into account inaccuracies in the digital-to-analog voltage conversion process. Another technical advantage of the present invention is that it allows a larger number of different monochrome actuation levels to be achieved using the same digital-to-analog conversion circuitry. Another technical advantage of the present invention is that it can accept different levels of digital specification of desired voltage. Another technical advantage of the present invention is that calibration across multiple digital-to-analog converters is consistent.
Other technical advantages of the present disclosure will be readily apparent to one skilled in the art from the following figures, descriptions, and claims. Various embodiments of the invention obtain only a subset of the advantages set forth. No one advantage is critical to the invention. For example, one embodiment of the present invention may only provide the advantage of controlling the pixels of a liquid crystal display, while other embodiments may provide several of the specified and apparent advantages.